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Agronomic Crops Network

Ohio State University Extension


Rhizobium Inoculation

Twenty-seven soybean inoculation trials were conducted between 2013 and 2014 in fields with a history of soybean production. Across the 27 trials (10 locations and six inoculant products), the average yield increase due to inoculant was 1.5 bushels per acre. Due to the relatively small yield increase associated with inoculant, we can only report the 1.5 bushel per acre yield increase at a 70 percent confidence level. The cost of inoculating an acre of soybeans is $4 to $5, depending on the product and rate used. If soybeans are worth $9 per bushel, the per acre profit for inoculating soybeans would be about $13 to $14 per acre. 

When loading a drill or planter using an auger, liquid or dry inoculation materials should be added to the seed as it enters the auger for thorough application. When loading a planter or drill from bags, fill the seed box to a depth of 3 inches and scatter an appropriate amount of inoculum over the seed and mix thoroughly. Continue to add seed in 6-inch layers, treating each until the box is filled. With some dry materials, it may be necessary to moisten seed slightly to increase adherence. A few small specks of inoculum on each seed is adequate. At the recommended use rate, there will be more than 500,000 bacterial cells on each seed. Excessive amounts of inoculum on seed can reduce seed metering by up to 35 percent. Seeding equipment should be calibrated using the treated seed to be planted. Some seeding rate monitors allow a continuous check of seeding rates so adjustments can be made to the seeding rate if and when necessary. 

When soybeans are planted in a field for the first time, it is not uncommon for even the most ideal inoculation procedures to be less than adequate for producing enough nitrogen for a good crop. When the nodules are insufficient to supply adequate nitrogen, it will be necessary to supply some nitrogen to the crop. In this event, one application of 75 pounds actual nitrogen as urea can increase yields by 8 to 12 bushels per acre. This supplemental nitrogen should not be applied until flowering, which is usually late June and July depending on variety maturity, date of planting, and the weather. To assure the establishment of a reliable inoculation for future years, it is advisable to grow soybeans in a new field two successive years and to inoculate the seed thoroughly both years. 

For satisfactory nitrogen fixation in eastern Ohio where soils tend to be more acid, the pH in the plow layer should be above 6.5, and the percent base saturation of calcium and magnesium should be greater than 40 and 10 percent, respectively. On fields where the lime requirement is very high, a shallow incorporation (2 to 4 inches) of 2 to 4 tons of dolomitic limestone will aid in the establishment of bacterial colonies on the root system. Dolomitic limestone should be used whenever magnesium levels are lower than 10 percent base saturation.